Corona ions may be generated from air by the application of high voltage to fine wires and sharp electrode tips (G. W. Trichel, Physical Review 54, 1078 (1938); R. Williams and A. Willis, J. Appl. Phys. 39, 3731-3736 (1968); M. Goldman, A. Goldman, and R. S. Sigmond, Pure & Appl. Chem. 57, 1353-1362 (1985); M. Pavlik and J. D. Skalny, Rapid Communications in Mass Spectrometry 11, 1757-1766 (1997)). The deposition of corona ions has been used for decades to apply electric fields to dielectrics and to other electronic material structures. Such methods avoid the fabrication of electrodes on the material surface. When combined with a Kelvin probe (K. Besocke and S. Berger, Rev. Sci. Instrum. 47, 840-842 (1976)), this noncontact approach permits the potential of the charged, or biased, surface to be determined. The combination has been used to study electronic trapping in materials, mobile ion density, and transport through dielectric films (R. Williams and A. Willis, J. Appl. Phys. 39, 3731-3736 (1968); G. W. Hughes, R. J. Powell, and M. H. Woods, Appl. Phys. Lett. 29, 377-379 (1976); R. Williams, Journal of Vacuum Science & Technology 11, 1025-1027 (1974); R. Williams, Journal of Vacuum Science & Technology 14, 1106-1101 (1977); R. Williams and M. H. Woods, J. Appl. Phys. 44, 1026-1028 (1973); R. Williams and M. H. Woods, Appl. Phys. Lett. 22, 458-459 (1973), R. Williams and M. H. Woods, J. Appl. Phys. 46, 695-698 (1975); M. H. Woods and R. Williams, J. Appl. Phys. 44, 5506-5510 (1973); M. H. Woods and R. Williams, J. Appl. Phys. 47, 1082-1089 (1976); P. Edelman, A. M. Hoff, L. Jastrzebski, and J. Lagowski, U.S. Pat. No. 5,773,989, 1998; A. M. Hoff, S. Aravamudhan, A. Isti, and E. I. Oborina, J. Electrochem, Soc. 154, H977-H982 (2007)). For example, tunneling due to substrate emission in oxide films on silicon can be accomplished by biasing the structures with deposited corona ions and measuring the resulting potential decay with a Kelvin probe (Z. A. Weinberg, W. C. Johnson, and M. A. Lampert, J. Appl. Phys. 47, 248-255 (1976); Z. A. Weinberg, Solid-State Electron. 20, 11-18 (1977); Z. A. Weinberg, J. Appl. Phys. 53, 5052-5056 (1982)). The need for in-line monitoring and control of manufacturing processes led to the application of these methods in the silicon integrated circuits industry (P. Edelman, A. M. Hoff, L. Jastrzebski, and J. Lagowski, U.S. Pat. No. 5,773,989, 1998; M. Wilson, J. Lagowski, A. Savtchouk, L. Jastrzebski, and J. D'Amico, in COCOS (Corona Oxide Characterization of Semiconductor) Metrology: Physical Principles and Applications, San Jose, Calif., 1999 (ASTM); D. K. DeBusk and A. M. Hoff, Solid State Technology 42, 67 (1999)).
Corona-Kelvin metrology, C-KM, is now in common use in integrated circuit manufacturing for noncontact and preparation-free characterization of dielectrics on silicon (M. Wilson, D. Marinskiy, A. Byelyayev, J. D'Amico, A. Findlay, P. Edelman, L. Jastrzebski, and J. Lagowski, Trans. ECS 11, 347-361 (2007)). The metrology involves three elements: (1) placement of a precise amount of electric charge on a dielectric surface as ions from a corona discharge in air; (2) monitoring the surface voltage change with a vibrating Kelvin probe; and (3) determination of the semiconductor surface barrier potential, VSB, separate from the dielectric potential, VOX. In the case of oxides on SiC, this metrology has been applied to the determination of the capacitance-voltage dependence (A. M. Hoff and E. Oborina, in Silicon Carbide and Related Materials 2006, Pts 1 and 2, edited by R. P. Devaty, D. J. Larkin, and S. E. Saddow (2006), p. 1035-1038; A. M. Hoff, E. Oborina, S. E. Saddow, and A. Savtchouk, in Silicon Carbide and Related Materials 2003, Pts 1 and 2; Vol. 457-460 (2004), p. 1349-1352) and Fowler-Nordheim characteristics (E. I. Oborina, H. Benjamin, and A. M. Hoff, J. Appl. Phys. in press (2009)) of as-grown dielectrics. However, there exists a problem applying Corona-Kelvin metrology to oxides on SiC. In particular, with the application of oxides on SiC, there exists a problem with the third part of the Corona-Kelvin metrology process. The third element is used to obtain dielectric charges, Qit, and the dielectric interface trap density, Dit. The specific zero value of the surface photovoltage identifies the flat-band condition at dielectric-silicon interface, where the flat-band condition is a reference in calculation of the silicon surface barrier, Vsb, and the barrier change upon corona charging. Corona charge at flat-band gives the total dielectric charge, Qtot. With silicon, the surface photovoltage can be found fairly easily with the use of a light with photon energy larger than the silicon energy gap of 1.1 eV, and a photon energy small enough to not cause any oxide charge changes. The problem with SiC is that the energy gap is 3 eV. This value is above the band gap illumination where significant changes would occur to the interface charge, Qit, and the dielectric trapped charge, Dit.
Starting from a defined initial condition of a dielectric-semiconductor structure, the automated sequential accomplishment of elements 1 and 2 of the Corona-Kelvin metrology determine the voltage-charge characteristics, V-Q, the capacitance-charge characteristics, C-Q, and the electrical thickness of the dielectric film on the semiconductor (M. Wilson, D. Marinskiy, A. Byelyayev, J. D'Amico, A. Findlay, P. Edelman, L. Jastrzebski, and J. Lagowski, Trans. ECS 11, 347-361 (2007)). To quantify the dielectric charges and the interface trap density in as-grown dielectric-silicon structures the third element can be implemented. The VSB value for each quantity of deposited charge on the surface during the measurement sequence is obtained from the difference between the total structure voltage determined in the dark and the structure voltage when the material is illuminated to null the band bending at the semiconductor surface. Following each illumination, a relatively short time is required in the silicon to establish the pre-illumination value of the total voltage once the light is turned off. Therefore, in a typical sequence of measurements on silicon, where the semiconductor is swept from accumulation to depletion, two V-Q curves are generated that correspond to: A) the dielectric voltage versus density of charge applied, light measurements; and B) the total voltage of the structure versus the charge applied, dark measurements. The difference between curves A and B corresponds to the VSB-Q characteristic. Further, characteristics A and B intersect at the flatband potential. In the case of the wide band gap material SiC, a recovery time for VCPD comparable to silicon following illumination is not experimentally observed.
Again, the specific zero value of VSB identifies the flatband condition at the dielectric-semiconductor interface. This flatband condition is a reference in calculations of the semiconductor space charge and the change in VSB induced by charging the dielectric surface with corona ions. For example, the quantity of corona charge needed to achieve flatband starting from the initial charge state of the oxide, gives the total dielectric charge, QTOT (M. Wilson, J. Lagowski, L. Jastrzebski, A. Savtchouk, and V. Faifer, in Characterization and Metrology for ULSI Technology; Vol. 550, edited by D. G. Seiler, A. C. Diebold, R. McDonald, W. M. Bullis, P. J. Smith, and E. M. Secula (AIP, 2001), p. 220-225), present in an as-grown film. In the case of silicon, VSB is easily driven to zero volts, independent of the charge density on the dielectric surface, using light with photon energy larger than the silicon energy gap of 1.1 eV, but at the same time a sufficiently low intensity is used to avoid photo-induced change of the oxide charge.
Accordingly, there is a need in the art for a method and apparatus for determining the interface trap charge and/or interface trap density of a semiconductor-dielectric or semiconductor-oxide interface, for wideband gap semiconductor and/or structures having charge centers (e.g., defects) that do not depopulate after illuminating and turning illumination off.